Effect of Plasma Scale Length on Multi-MeV Proton Production by Intense Laser Pulses

Abstract

The influence of the plasma density scale length on the production of MeV protons from thin foil targets irradiated at I lambda(2) = 5 x 10(19) W cm(-2) has been studied. With an unperturbed foil, protons with energy >20 MeV were formed in an exponential energy spectrum with a temperature of 2.5+/-0.3 MeV. When a plasma with a scale length of 100 microm was preformed on the back of the foil, the maximum proton energy was reduced to <5 MeV and the beam was essentially destroyed. The experimental results are consistent with an electrostatic accelerating mechanism that requires an ultrashort scale length at the back of the target.

Investigation of the interaction of a laser pulse with a preformed Gaussian Sn plume for an extreme ultraviolet lithography source

“…A comparison between cases with and without a prepulse shows a variation in ion energies of a factor of 2-4. 7 Numerical simulations also suggested approximately two times reduction in velocity when a smooth initial ion density profile is considered. 5 Recently we demonstrated a much larger reduction factor of 30 in ion energy from a laser-produced Sn plasma by introducing a low energy prepulse.…”

“…It has been shown that both the maximum energy and angular distribution depend on the level of the prepulse. 7,8 Ion acceleration is more efficient with a short initial scale length. A comparison between cases with and without a prepulse shows a variation in ion energies of a factor of 2-4.…”

Plasma Jets Driven by Ultraintense-Laser Interaction with Thin Foils

“…Some further considerations can be made regarding the role played by the prepulse. The observation of energetic protons in both experiments implies that the prepulse was not capable of eroding the total target thickness before the arrival of the main CPA on target [21]. Indeed, 2D hydrodynamic simulations using the code POLLUX [22] indicate that ablation by a prepulse with the experimental parameters is not sufficient to disrupt the rear surface of foils thicker than 1 m. A combination of the hydrosimulation results and interferometric measurements also provides information on the plasma conditions at the front surface immediately before the CPA interaction: the preplasma has a gradient of a few m scale length at the critical density surface, which is located a few m in front of the solid density target.…”

Enhanced proton beams from ultrathin targets driven by high contrast laser pulses

“…7,8 An induced density gradient can drastically reduce the effectiveness of the TNSA mechanism. 9 The thinner the target, the more sensitive it is to such prepulses. For a given peak laser intensity, the optimum target thickness is thus related to the laser temporal contrast, defined as the ratio between the intensity of the main pulse to that of the pedestal or prepulse.…”